In recent years, for the purpose of reducing fuel consumption due to saving body weight of automobiles and ensuring safety upon collision, demands for high-strength steels have increased more and more. Accordingly, steel sheets having a tensile strength of 980 MPa or higher class have been required in place of those having a tensile strength of 590 MPa class. Moreover, in the case of high-strength steel sheets having a tensile strength of 980 MPa or higher class, deterioration of formability cannot be avoided and there was restriction on applications since it is possible to apply to parts having complicated shapes. In applications where the steel sheet is press-formed into a complicated shape, it is required to provide a high-strength steel sheet having both elongation and stretch flangeability.
Now various steel sheets including residual austenite in the metal structure are put into practical use as high-strength steel sheets that exhibit excellent elongation.
For example, Non-Patent Document 1 discloses a steel sheet in which a bore expansion property (i.e. stretch flangeability) is enhanced while ensuring a high strength by constituting the metal structure with a composite structure which mainly contains bainitic ferrite and also contains lath-type residual austenite. However, when a tensile strength (TS) becomes a tensile strength of 980 MPa or higher class, this steel sheet shows TS×El as an indicator of the strength (TS) and ductility (El) of 9,000 to 10,300 at most and therefore it is hardly to say that the steel sheet is satisfactory.
It is considered that, in a mass production line of a practical operation using a continuous annealing furnace, a maximum heating temperature is about 900° C. and a heating time is 5 minutes or less. However, under the production conditions disclosed in this document, it is required to cool to a temperature within the range from 350 to 400° C. in a salt bath after annealing at 950° C. for 1,200 seconds, and thus this method is not suited for the practical operation.
In Patent Document 1, elongation of about 20% and stretch flangeability (λ) of 55% are attained while ensuring a tensile strength of 980 MPa or higher by constituting a matrix phase with a structure composed mainly of bainitic ferrite and 3% or more of residual austenite. However, in this technique, the addition of expensive alloy elements such as Mo, Ni and Cu is indispensable and it leaves a room for improvement in cost.
Furthermore, Patent Document 2 discloses steel sheets having enhanced total elongation and stretch flangeability by mainly constituting a matrix structure with tempered bainite. However, since a study is mainly made on steels having a 900 MPa class tensile strength in this steel type, delayed fracture, which is caused in steels having a tensile strength of 980 MPa or higher class, is not sufficiently studied.    Non-Patent Document 1: ISIJ International, Vol. 40 (2000), No. 9, pp. 920 to 926    Patent Document 1: Japanese Unexamined Patent Publication (Kokai) No. 2004-332099    Patent Document 2: Japanese Unexamined Patent Publication (Kokai) No. 2002-30933